In the technical field of radio communication, it is discussed that a reflectarray for implementing scattering of an incident wave toward an arbitrary direction is applied to ensure a communication area or for other purposes. Also, the reflectarray may be used to form multiple paths in a line-of-sight propagation environment where a direct wave is dominant to improve throughput and/or reliability in a Multiple Input Multiple Output (MIMO) scheme.
In addition, there are some cases where two mutually orthogonally polarized waves are used in communication as polarization diversity or polarization MIMO for implementation of higher speed and larger capacity of communication. In these cases, the polarization is linear polarization and may be referred to as an electric wave (Transverse Electric wave: TE wave) having an electric field component vertical to a plane of incidence and an electric wave (Transverse Magnetic wave: TM wave) having an electric field component in parallel to the plane of incidence, for example. Alternatively, the polarization may be referred to as a vertical polarization wave having an electric field component vertical to the ground and a horizontal polarization wave having an electric field component in parallel to the ground. Also, an electric field rotates in various directions in an outdoor location due to affection of propagation environment. In this case, the electric field may be considered to have two components, that is, a vertical component and a horizontal component. In any of the cases, two planar waves, amplitude directions of whose electric fields are mutually orthogonal, are available in communication. However, conventional reflectarrays are difficult to reflect two polarized waves arriving from a certain direction to respective different directions as desired.
On the other hand, according to a radio communication system such as a LTE (Long Term Evolution) Advanced scheme, multiple frequency bands or carriers are used in communication as needed. Accordingly, it is desirable that a reflectarray for reflecting a wave for use in communication also corresponds to the multiple frequency bands (multiband). Some conventional reflectarrays supporting the multiband are described in Non-Patent Document 1. A reflectarray as described in Non-Patent Document 1 has a broken circular element for Ka band (32 GHz), a broken rectangular linear element for X band (8.4 GHz) and a cross dipole element for C band (7.1 GHz). However, this reflect array is targeted to circular polarization and is unavailable for direct polarization without modification. In addition, the reflectarray as described in Non-Patent Document 1 must be processed to have a complicated element shape such that it can operate appropriately in Ka, X and C bands, which can increase the cost.
A conventional reflectarray uses an about ½ wavelength element such as a macrostrip element as described in Non-Patent Document 2. By changing the size of this element, the reflection phase can be changed with misalignment of a resonant frequency. Thus, the phase of each array element may be determined such that the planar wave is oriented to a desired direction. It has been reported that a cross dipole can be used to implement such a reflectarray for associating ½ wavelength elements with multiple polarized waves and reflecting two polarization waves arriving from a certain direction to respective desired directions (see Non-Patent Documents 3 and 4).
Meanwhile, a reflectarray using a mushroom structure much smaller than the wavelength has been reported as a method for controlling the reflection direction with a wider angle than a reflectarray using conventional ½ wavelength elements (Non-Patent Document 5). However, no mushroom structure available in dual use for orthogonally polarized waves has existed. Accordingly, no mushroom structure that can achieve wide angle control in dual polarization has existed.
In a radio communication system such as the LTE-Advanced scheme, on the other hand, multiple frequency bands or carriers are used in communication as needed. Accordingly, it is desirable that a reflectarray for reflecting waves for use in communication also supports multiple frequency bands (multiband). Some conventional reflectarrays supporting the multiband are described in Non-Patent Documents 1 and 3 below. A reflectarray as described in Non-Patent Document 1 has a broken circular element for Ka band (32 GHz), a broken rectangular linear element for X band (8.4 GHz) and a cross dipole element for C band (7.1 GHz). A reflectarray as described in Non-Patent Document 3 uses a cross dipole as an element to determine the reflection phase by changing the length of the cross dipole element with respect to the X direction for an incident wave of a first frequency f1 having an electric field in parallel to the X-axis and determine the reflection phase by changing the length of the cross dipole element with respect to the Y direction for an incident wave of a second frequency f2 having an electric field in parallel to the Y-axis.
However, the conventional structure is based on a ½ wavelength element and is difficult to apply for angle control wider than 40 degrees due to occurrence of grating lobe and influence of mutual coupling between elements.
In order to overcome these problems, reflectarrays having mushroom structures as described in Non-Patent Documents 5 and 6 have been proposed. However, these are not dual polarization elements. Accordingly, it is difficult to design the reflectarray independently for individual polarization waves. Thus, it can be seen that when a Y directional gap gy between mushrooms changes, the reflection phase value would also change for a X directional gap gx between the mushrooms.